18 outlandish conspiracy theories Donald Trump has floated on the campaign trail and in the White House

In May 2011, reporters swarmed now-President Donald Trump as he exited the Hyatt in Washington, DC, after the White House Correspondents' Dinner.

Many wanted a response from Trump, who had just watched President Obama deliver jokes that night about Trump's constant questioning of the legitimacy of Obama's birth certificate.

Over six years later, Trump is still not convinced of the legitimacy of Obama's birth certificate. But this was perhaps the first of numerous debunked or unverified conspiracy theories that Trump has entertained during his time in the political spotlight.

Throughout the 2016 campaign and while in the White House, Trump has floated theories fueled by the conspiratorial-minded corners of supermarket tabloids and the internet, something unprecedented in modern politics. He's often used them as weapons against his opponents.

Here are 18 of the most notable conspiracy theories Trump has entertained:

Questions about President Obama's birth certificate.

While mulling a potential 2012 presidential bid, Trump became the most high-profile figure to promote the rumors suggesting that President Obama was not born in the US.

Trump claimed he'd deployed private investigators who "could not believe what they're finding" about Obama's place of birth.

He also repeatedly clashed with reporters who pushed him on the issue. During one contentious interview, he told ABC's George Stephanopoulos that he'd been "co-opted" by "Obama and his minions" when the anchor tried to push back on Trump's claims.

"He had intimate knowledge of what was going on," Trump said of Foster's role in the White House. "He knew everything that was going on, and then all of a sudden he committed suicide."

He added: "I don’t bring [Foster’s death] up because I don’t know enough to really discuss it. I will say there are people who continue to bring it up because they think it was absolutely a murder. I don’t do that because I don’t think it’s fair."

Questions about whether Syrian refugees are ISIS terrorists.

Donald Trump.AP

Trump has, in part, justified his plan to temporarily bar Muslim immigrants from entering the US by claiming that refugees coming from Syria "could be a Trojan horse."

"It could be one of the greatest coups of all time," Trump told Fox News' Sean Hannity last year. "They could be ISIS. It could be a plot. I mean, I don't want to think in terms of conspiracy, but it could be a plot."

But the process for vetting refugees typically lasts 18 to 24 months, and immigration experts maintain it is one of the most difficult ways for terrorists to attempt to enter the US legally.

"It is extremely unlikely that someone who is a terrorist will be sent through the refugee resettlement program," Greg Chen, the director of advocacy at the American Immigration Lawyers Association, told Business Insider.

He added: "It takes a great deal of time, and it wouldn't make sense for someone who is a terrorist for someone to go through that process. There are going to be easier ways for a terrorist to try to infiltrate, rather than going through the refugee resettlement program."

Questions about whether an ISIS-linked terrorist attempted to charge at Trump on stage.

Secret Service agents form a perimeter around Trump after a man attempted to charge the stage at an Ohio rally.reuters

After an attendee at Trump's March rally in Dayton, Ohio, attempted to charge the stage, Trump claimed a video he retweeted proved the attendee was a terrorist linked to ISIS.

"He was playing Arabic music. He was dragging the flag along the ground, and he had internet chatter with ISIS and about ISIS. So I don't know if he was or not," Trump said. "But all we did was put out what he had on his internet. He's dragging the flag, the American flag, which I respect obviously more than you."

He added: "What do I know about it? All I know is what's on the internet. And I don't like to see a man dragging the American flag along the ground in a mocking fashion."

Questions about Supreme Court Justice Antonin Scalia's death.

Asked about the circumstances of Scalia's death, Trump said he was unsure about what caused Scalia's death. Trump noted a pillow was found over the justice's face, a claim authorities rebutted.

"I'm hearing it's a big topic," Trump said in a radio interview. "It's a horrible topic but they're saying they found the pillow on his face, which is a pretty unusual place to find a pillow."

He added: "I can't give you an answer. It's just starting to come out now."

Questions about whether childhood vaccines cause autism.

Donald Trump.REUTERS/Scott Audette

At a Republican presidential debate last year, CNN host Jake Tapper asked Trump about his position that vaccines can cause autism.

"We had so many instances, people that work for me, just the other day, 2 years old, a beautiful child, went to have the vaccine and came back and a week later got a tremendous fever, got very, very sick, now is autistic," Trump said.

Shortly after Trump's assertion, former presidential candidate and neurosurgeon Ben Carson corrected the real-estate mogul, pointing out that overwhelming medical evidence suggests that there's no link between autism and vaccines.

A 2013 study from the Centers for Disease Control and Prevention found no connection between vaccines and an increased risk of autism.

Questions about whether Muslims in New Jersey were cheering after 9/11.

Donald Trump.Justin Sullivan/Getty Images

For weeks last year, Trump emphatically claimed he saw televised news reports of Muslims cheering in New Jersey after the September 11, 2001, terrorist attacks.

"There were people that were cheering on the other side of New Jersey, where you have large Arab populations. They were cheering as the World Trade Center came down," Trump said during an ABC interview.

He added: "I know it might be not politically correct for you to talk about it, but there were people cheering as that building came down — as those buildings came down. And that tells you something. It was well-covered at the time."

Questions about whether wives of 9/11 hijackers fled to Saudi Arabia before the attacks.

Donald Trump.Joe Raedle/Getty Images

The presumptive Republican presidential nominee repeatedly stated last year that the terrorists who carried out the September 11, 2001, attacks moved their families out of the US to Saudi Arabia several days before the hijacking.

"When you had the World Trade Center go, people were put into planes that were friends, family, girlfriends, and they were put into planes and they were sent back, for the most part, to Saudi Arabia," Trump said. "They knew what was going on. They went home and they wanted to watch their boyfriends on television."

The 9/11 commission report, the most extensive investigation into the events surrounding the attacks, determined that few of the hijackers kept in contact with their families, and none had family members living in the US.

Questions about the legitimacy of climate change.

Donald Trump at his golf course in Scotland.REUTERS/David Moir

Though many Republican leaders remain skeptical of climate change, Trump has taken his skepticism a step further. In 2012 he suggested that climate change is a "total, and very expensive hoax" perpetuated by China's government.

"The concept of global warming was created by and for the Chinese in order to make U.S. manufacturing non-competitive," Trump tweeted in 2012.

Trump backed off the tweet, telling Fox News in January that his comment was a "joke." Still, the real-estate mogul has repeatedly maintained that climate change was a hoax, and said climate-change studies are "done for the benefit of China."

According to NASA, 97% of publishing climate scientists believe that human activities such as burning of fossil fuels have caused climate change.

Questions about whether asbestos is a "great con."

Donald Trump at the New York Stock Exchange in 1995.AP Photo/Kathy Willens

When confronted on ABC's "This Week" about whether he believed Rubio was not constitutionally permitted to occupy the presidency, Trump, whose mother was born in Scotland, refused to disavow the tweet.

"I've never looked at it, George," Trump said of the tweet. "I honestly have never looked at it. As somebody said, he's not. And I retweeted it. I have 14 million people between Twitter and Facebook and Instagram, and I retweet things and we start dialogue and it's very interesting."

He added: "I'm not sure. Let people make their own determination."

Questions about Fox News being owned by a Saudi billionaire.

Donald Trump

Trump's war with Fox News' Megyn Kelly recently reached a detente.

But during the peak of Trump's rhetorical battle with Kelly, he perpetuated a prominent outlandish theory from one of his Twitter followers.

In January, the real-estate mogul retweeted a photo purportedly showing Saudi billionaire Prince Alwaleed bin Talal with Kelly. The photo claimed the prince was a partial Fox News owner, which multiple outlets found was untrue. Alwaleed's investment company owns a small share of 21st Century Fox.

Questions about the legitimacy of the "Access Hollywood" tape

Trump in the "Access Hollywood" tape.NBC

Toward the tail end of his presidential campaign, the 2005 "Access Hollywood" tape featuring Trump apparently admitting that he likes to grab women "by the p----" received broad coverage, and Trump apologized for his comments shortly afterward.

More recently though, after various allegations of sexual harassment in media and politics have begun to surface, Trump has walked back these comments.

“We don't think that was my voice,” Trump reportedly told a senator, according to The New York Tiimes.

The Times' sources did not elaborate on why Trump has begun to doubt the authenticity of the tape's audio.

Claims that Joe Scarborough killed one of his interns

Joe Scarborough on MSNBC.MSNBC

In a tweet Trump sent Wednesday morning, he made references to a conspiracy theory that claims MSNBC anchor Joe Scarborough of "Morning Joe" murdered one of his staffers in Florida in 2001.

"So now that Matt Lauer is gone when will the Fake News practitioners at NBC be terminating the contract of Phil Griffin?" the tweet read. "And will they terminate low ratings Joe Scarborough based on the 'unsolved mystery' that took place in Florida years ago? Investigate!"

While Scarborough was serving as a Republican congressman in Florida's 1st district, one of his interns, Lori Klausutis, was found dead in the office. A coroner found no evidence of foul play, and indicated that the death occurred because of a heart problem that caused the intern to fatally hit her head on her desk.

Claims that Obama had wiretapped Trump's phone

Obama campaigns in support of Northam at a rally with supporters in Richmond, VirginiaThomson Reuters

In March, Trump sent a tweet accusing Obama of wiretapping his phones in Trump Tower.

"Terrible!" Trump wrote, "Just found out that Obama had my 'wires tapped' in Trump Tower just before the victory. Nothing found. This is McCarthyism!"

PolitiFact and other outlets have debunked the claim. An Obama spokesman also issued a response to the allegation, saying: "Neither Barack Obama nor any White House official under Obama ever ordered surveillance of any U.S. citizen."

Claims that voter fraud in the 2016 election cost him the popular vote

People vote at the Evergreen Recreation Center during the 2016 presidential election in the Boyle Heights area of Los Angeles.Thomson Reuters

In a tweet sent shortly after the November 2016 election, Trump wrote: "In addition to winning the Electoral College in a landslide, I won the popular vote if you deduct the millions of people who voted illegally."

This unsubstantiated claim was repeated by Kris Kobach, the chairman of Trump's panel on voter fraud, in July. The fact-checking site Snopes has debunked the claim entirely, citing "zero evidence."

Britain First has frequently targeted mosques and Muslims in the UK in order to brand all Muslims as violent extremists, and Trump's retweet of the videos was widely seen as a tacit endorsement of the group's efforts.

Although the authenticity of the videos has been called into question, White House press secretary Sarah Huckabee Sanders has maintained this doesn't matter.